Radio communications system designed for a low-power receiver

ABSTRACT

The invention relates to methods by which radio signals can be transmitted to, and received by, a radio receiver such that the receiver consumes very little power from a battery or energy source. The invention also relates to methods by which radio signals, modulated with data information, can be produced by a transmitter that consumes very little power. The invention is applicable not only to medical implants, but any application requiring a radio receiver to operate with very low power consumption.

This application is a continuation in part of U.S. patent applicationSer. No. 11/877,821, filed 24 Oct. 2007, the specification of which ishereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a low power radio frequency receiver, amatching transmitter, a communication method for a low power receiver,and an electromagnetic signal.

2. Description of the Related Art

State-of-the-art battery powered medical implants must not only providethe prescribed therapy, but they must also be as small as possible,extremely reliable, and have a long service life. For example, theservice life of cardiac pacemakers and implantable cardioverterdefibrillators (ICDs) is expected to be seven years or more. Since allelectronic devices consume electrical energy, medical implants withbatteries that are not readily accessible, or easily replaced, mustoperate with minimal power consumption. Batteries in medical implantshave a small and finite capacity to provide electrical energy, and assuch, the operational longevity of devices being powered by them isdependent upon the rate at which the devices consume energy. Sinceimplanted medical devices are required to operate for many years from alimited source of energy, the electronic circuitry in the devices mustbe designed to operate with minimal power consumption.

Present day medical implants often utilize radio telemetry tocommunicate patient medical data and implant status to a physician. Thistelemetry may occur frequently, as in the case of home monitoringapplications, or it may occur less frequently, such as during patientfollow-up examinations in a physician's office. In any event, the radiotelemetry circuitry in the implant consumes battery power, and the henceimpacts the longevity of the implant.

In the case of pacemakers and ICDs, the primary purpose of the implantis to regulate a patient's heartbeat and/or provide a life-saving shockto treat ventricular fibrillation. The implant's telemetry function isof secondary importance, and because of this, the battery power consumedby the radio telemetry circuitry must be minimal and have negligibleimpact on the service life of the implant.

In many battery-powered applications, the architectures of devices mayemploy special techniques to minimize power consumption. For example,devices may be designed to operate in several different modes, some ofwhich may consume less power than others. Devices may have a “sleep”mode whereby they consume very little power or no power at all, and thenone or more operational modes where the devices have more functionalcapability—at the expense of consuming more power. When operating inthis manner to save battery power, the devices may change from oneoperational mode to another. Changes in operational mode may bedetermined by a time-sequence under the control of hardware or software.Changes in operational mode may also occur upon reception of a signal,or stimulus from an external device or sensor, or they may result fromthe reception of radio frequency commands transmitted to the device.

In a battery-powered device that changes operating modes upon thereception of radio frequency commands, the radio receiver in the devicemust be powered on, and remain on continuously, or nearly continuously,to ensure the device receives commands, and responds appropriately andin a timely fashion. The time of arrival of a transmitted command isusually not know by the device in advance, and so the receiver must bepowered on in anticipation that a command may be transmitted to it atany time. As a consequence, the device's radio receiver willcontinuously consume power from the battery even when commands are notbeing transmitted to it.

The radio frequency electronics circuitry in a conventional receivertypically consumes a considerable amount of power, and hence theoperational longevity of a battery-powered device is significantlyreduced as the receiver remains powered on for long periods of time.Because a patient must undergo a surgical procedure to replace animplant when its battery power is nearly depleted, it is highlydesirable to minimize the power consumption of radio frequency receiversemployed in medical implants.

BRIEF SUMMARY OF THE INVENTION

This invention describes methods by which radio signals can betransmitted to, and received by, a radio receiver such that the receiverconsumes very little power from a battery or energy source. Theinvention is applicable not only to medical implants, but anyapplication requiring a radio receiver to operate with very low powerconsumption.

According to a further aspect, the invention facilitates implementing aradio communications system that features a wireless transceiver thatconsumes far less power than traditional transceiver architectures.Because the transceiver consumes far less power, it is particularlyattractive for use in battery-operated medical implants such aspacemakers and ICDs.

Description of the Receiver

According to the invention, the requirements for a low-power receiverare fulfilled by a device comprising: at least one antenna, a resonantcircuit, and a nonlinear device. Said antenna forms a part of theresonant circuit or it is directly or indirectly coupled to the resonantcircuit, according to the invention.

Said antenna and resonant circuit are tuned to receive a first highfrequency carrier of a predetermined first frequency F1, and a secondhigh frequency carrier of a predetermined second frequency F2simultaneously, which differ from each other by a predeterminedintermediate or baseband frequency Fout=|F2−F1|.

Furthermore, a nonlinear device is directly or indirectly connected tothe resonant circuit in a manner to cause mixing, or heterodyning, ofthe two carrier frequencies (F1 and F2), so as to produce anintermediate frequency (IF) or baseband signal Fout.

The low power receiver is adapted to operate according to theheterodyning method of down converting a high frequency signal to anintermediate frequency (IF) signal, or baseband frequency signal, bymixing the first high frequency carrier and the second high frequencycarrier. The intermediate frequency signal, or baseband signal, producedby the mixing or heterodyning behaviour of the said nonlinear device,may be used directly as a signal to other electronic circuitry in thedevice, or the signal may be further demodulated to extract otherinformation that may be contained in the resultant IF signal.

The low power receiver thus operates as a heterodyning receiver usingthe second high frequency carrier to replace the otherwise requiredlocal oscillator signal in a traditional super-heterodyne receiverarchitecture. This has the advantage that a very simple receiver designis used for reception of a high frequency carrier. Due to the simpledesign, which contains very few components, the power consumption of thereceiver is very low.

Although the frequencies of the high frequency carriers may be veryhigh, the frequency of the IF or baseband signal may be very low, if thedifference between the frequencies of the high frequency carriers issmall. In other words the frequencies of the high frequency carriers canbe very close in value. Since the IF or baseband frequency is very low,numerous high frequency components and circuitry can be eliminated inthe design and implementation of the low power receiver. For example,compared to a traditional super-heterodyne receiver that receives asingle high frequency carrier, the following components can beeliminated: RF amplifiers, the local oscillator circuitry (and phaselocked loop circuitry if a PLL is used), the local oscillator bufferamplifiers, and interstage impedance matching networks.

The resultant output signal may be an intermediate frequency (IF)signal, or it may be a baseband signal, and it may be the desiredinformation to be received. In the case where the output signal is an IFsignal, the IF signal may be further demodulated by subsequent stages toproduce a baseband frequency signal that may contain the desiredinformation to be received. The low power receiver thus is adapted tooperate in a heterodyning fashion using the transmitted second highfrequency carrier to replace an otherwise required local oscillatorsignal that is needed in a traditional super-heterodyne receiver.

In some traditional super-heterodyne receivers more than oneintermediate frequency is used. In these multi-conversion receivers,different IF stages with different intermediate frequencies exist toobtain a better signal-to-noise ratio, improved image rejection, andfiner tuning resolution. This requires additional local oscillators,mixers, and IF amplifier stages. If the application warrants it, the lowpower receiver can also employ additional IF stages similar to themethods used for traditional multi-conversion super-heterodynereceivers.

Another advantage of the invention is that because the low-powerreceiver's architecture is considerably simpler than a traditionalsuper-heterodyne receiver, it uses fewer components, and hence thereceiver can be made considerably smaller in size.

According to the invention, the intermediate frequency signal, orbaseband signal, is received from a transmitter within a pair of highfrequency carriers because the two high frequency carriers alreadycontain the intermediate frequency, or baseband signal, in thesuperposition of two high frequency carriers. The intermediatefrequency, or baseband signal, can also be seen as the beat frequencyresulting from the mixing or heterodyning of the two high frequencycarriers.

Since both high frequency carriers are received by the antenna, the highfrequency carriers are superimposed within the antenna. The receivedsecond high frequency carrier signal replaces the locally generatedlocal oscillator signal in a typical super-heterodyne receiver, and thenonlinear device in the invention replaces the mixer section used in atraditional super-heterodyne receiver.

In one embodiment of the invention, the low power receiver is adapted toconvert the high frequency carriers to an intermediate frequency signal,and then demodulate the intermediate frequency signal with a demodulatorso as to convert the IF signal into a baseband signal. The conversion ofthe high frequency carriers to the intermediate frequency signal is donesimply by receiving the two high frequency carriers and mixing, beating,or heterodyning them in the nonlinear device.

The two high frequency carriers are both received by the antenna. Theantenna is tuned to a middle frequency between the first and second highfrequency carrier. The superposition and heterodyning of the two highfrequency carriers produces the intermediate frequency signal. Thus theconversion from the high frequency carriers to the intermediatefrequency signal results from the superposition and mixing, beating, orheterodyning that automatically occurs during reception.

To obtain an intermediate frequency signal in a traditionalsuper-heterodyne receiver, it is necessary to mix the received highfrequency carrier with a signal that is locally generated in thereceiver at a different second frequency. The super-heterodyne receiver,as known in the art, therefore contains a local oscillator, whichproduces the second frequency. The intermediate frequency signal has afrequency equal to the difference between the carrier frequency and thefrequency of the local oscillator. A mirror frequency, or imagefrequency, equal to the sum of the carrier frequency and the localoscillator frequency is also produced in the heterodyning of twosignals. The process of producing the IF signal that is equal to thedifference of the two high frequency signals is referred to asdown-conversion. The process of producing the IF signal equal to the sumof the two high frequency signals is referred to as up-conversion.

In another embodiment of the invention, the low power receiver isadapted to convert the high frequency carriers directly to a basebandfrequency signal. This baseband signal can then be used directly as acontrol signal by other sections of the device, or it can contain thedesired communications signal information, or data, that was transmittedto the device.

In contrast to a traditional quadrature-amplitude modulation receiver,where two carriers of the same frequency, with a constant phasedifference of π/2, are used, the low power receiver according to theinvention receives two high frequency carriers at two differentfrequencies. A receiver for quadrature-amplitude modulated signals canalso work with the super-heterodyne method, but has the samedisadvantages, because it requires a local oscillator signal to obtainan intermediate frequency signal.

A high frequency signal, when used for transmission, is referred to ashigh frequency carrier. A high frequency carrier can be modulated withanother signal, or may not be modulated, i.e. it may be just acontinuous wave (CW) signal.

In a first embodiment of the high frequency carrier implementation forthe low power receiver, the first and second high frequency carriers arenot modulated (i.e they are CW carriers). The low power receiver isadapted to mix the two unmodulated carriers to obtain an intermediatefrequency signal, or the baseband frequency signal. The resultingintermediate signal, or baseband signal, contains the desiredinformation (that is, the data representing information) to be received.For example, the presence, or absence, of an intermediate frequency orbaseband signal is, in itself, is a binary representation of datarepresenting the desired signal information.

In a second embodiment of the high frequency carrier implementation forthe low power receiver, the first or the second high frequency carrieris a reference carrier, which is not modulated (i.e. a CW carrier), andthe other high frequency carrier is modulated with a data signal. Thelow power receiver is adapted to mix the unmodulated CW referencecarrier with the modulated high frequency carrier to obtain a modulatedintermediate frequency signal, or baseband frequency signal, thatcontains the data signal.

In a yet another embodiment of the high frequency carrier implementationfor the low power receiver, the first and the second high frequencycarriers are modulated with a data signal. The resultant intermediatefrequency signal, or baseband signal, will contain the data signals thatwere modulated onto the high frequency carriers. The signal informationis regained by demodulating the received intermediate frequency orbaseband signal. This has the advantage that one of the high frequencycarriers replaces an otherwise needed local oscillator signal, fromwhich the intermediate frequency is produced.

In a first embodiment of the antenna configuration, the receivercomprises two separate antennas each tuned to receive only one of thehigh frequency carriers. The receiver then combines and mixes, orheterodynes, the high frequency carriers to produce the intermediatefrequency or baseband signal.

In a second embodiment of the antenna configuration, the receivercomprises only one antenna with which it receives the two high frequencycarriers. In this embodiment the two high frequency carriers arecombined and superimposed in the antenna. The receiver mixes, orheterodynes, the already combined high frequency carriers to produce theintermediate frequency or baseband signal.

It should be noted that the amplitudes of the transmitted high frequencycarriers may be equal or different. If the antenna or the resonantcircuit is tuned to the first high frequency carrier, the receivedsignal may be higher than for the second high frequency carrier. In thiscase it may be useful to transit a greater amplitude or stronger signalfor the second high frequency carrier to obtain adequate signalamplitudes for both high frequency carriers within the receiver.

In one embodiment of the invention the low power receiver comprises afirst band-pass filter, which is connected directly, or indirectly, tothe output of the antenna. In this embodiment the first band-pass filterhas a pass band that is corresponds to the carrier frequencies. Theband-pass filter will attenuate unwanted signals at frequencies outsidethe frequency band of the desired transmitted signals. Attenuatingundesired signals, by filtering, will reduce the potential forinterference and improve the quality and reliability of the desiredcommunications link.

It should be noted that the first band-pass filter may also have apass-band around a frequency between the first and second high frequencycarrier frequencies. In this case both high frequency carriers can passthrough the band-pass filter with similar attenuation. It is desirablethat the filter's attenuation to the desired high frequency carriers beas low as possible.

It is also possible that the first band-pass filter may be tuned to onehigh frequency carrier frequency, and the antenna may be tuned to thefrequency of the same high frequency carrier. The attenuation for thishigh frequency carrier is lower then for the other high frequencycarrier.

It should be noted that more than one first band-pass filter can beemployed to obtain steeper roll-off and greater attenuation for signalsoutside the filter's pass band. Each high frequency carrier can have arespective band-pass filter.

In a further embodiment of the invention, the low power receiverincorporates a second band-pass filter, or a low-pass filter, that isconnected to the output of the nonlinear device. The second band-passfilter, or low-pass filter, has a pass band corresponding to theintermediate frequency or the baseband frequency. The second band-passfilter, or low-pass filter, is used to eliminate or attenuate the highfrequency carrier signals and unwanted signals resulting from themixing, or heterodyning, process.

In still another preferred embodiment, the second band-pass filter, orlow-pass filter, is connected to an intermediate frequency (IF) orbaseband amplifier. In this embodiment the low power receiver is adaptedto filter and to amplify the desired IF or baseband signal. In thisembodiment, the IF or baseband amplifier is used to obtain a highersignal level for the further processing of the received signal. Thefilter and amplifier may also serve to provide impedance matchingbetween sections of the receiver for maximum signal power transfer. Theamplifier may also have a specific frequency response and operate in afashion similar to a bandpass filter, or low-pass filter, to furtherattenuate unwanted signals.

In a further preferred embodiment of the invention the nonlinear deviceor the baseband amplifier is connected to a level detector circuit. Inthis embodiment the low power receiver is adapted:

-   1. to analyse the level of the IF or baseband signal, or the    filtered and amplified IF or baseband signal, with the level    detector circuit, and-   2. to produce a wakeup signal depending on the level of the IF or    baseband signal, or the filtered and amplified IF or baseband    signal, with the level detector circuit.

This embodiment of the invention facilitates implementing a low powerreceiver that can be operated continuously, nearly continuously, or beturned on for long intervals of time, while consuming minimal power froma battery. When the low power receiver detects a control signal from anexternal transmitter, it can wake up other circuitry in the device, orenable other programmed operating routines, that may have beenpowered-off to conserve battery power. The circuitry and/or operatingroutines can encompass device functions that require greater powerconsumption from the battery, such as data communications circuitry,processor circuitry, or the like.

In one embodiment of the invention, the signal from the output of thenonlinear device, the IF output, or baseband amplifier output, isconnected to a data detection circuit. The low power receiver is adaptedto convert the IF or baseband signal, or the filtered and amplified IFor baseband signal, with the data detection circuit, into a digital datasignal, and to output the digital signal to other circuitry forsubsequent processing. This data signal can be used to conveyinformation to the device such as identification information, operatinginstructions, or software or firmware updates.

Another embodiment of the low power receiver, according to theinvention, comprises a high frequency amplifier, which is connected tothe antenna or the resonant circuit. The high frequency amplifier isadapted to amplify the first and second high frequency carriers. Thehigh frequency amplifier may also provide filtering of the desired highfrequency carrier signals to reduce interference from unwanted signalsin the adjacent radio spectrum.

Another possibility is that the high frequency amplifier is connectedbetween the first band-pass filter and the nonlinear device. In thiscase the amplifier feeds the bandpass filtered and amplified signal tothe nonlinear device.

In the case where the low power receiver contains two antennas, or tworesonant circuits, there may be two or more high frequency amplifiersemployed, each connected to one antenna or resonant circuit.

In another embodiment of the invention, the low power receiver isadapted to receive more than two high frequency carriers at differentfrequencies, and to obtain an output signal from intermodulationproducts produced by nonlinear mixing of the high frequency carriers. Inthis case the output signal is generated by nonlinear mixing of highfrequency carriers, i.e. Fout=(nF1±mF2±pF3± . . . ) where n, m, p, . . .are integers, and F1, F2, F3 are high frequency carriers.

It should be clear that if more than one intermediate frequency is to bereceived, there may be several band-pass filters applied in parallel.

In a preferred embodiment, the nonlinear device is a single diode, anarray of diodes, a switching device, or a device with a strong 2^(nd)order transfer function.

There are several methods that can be used to obtain a device with theneeded 2^(nd) order transfer function for mixing, beating, orheterodyning. It should be noted that a device exhibiting a strongsecond-order transfer function provides an efficient mechanism forconverting two transmitted RF signals to an intermediate frequencysignal or baseband signal. Alternatively, any ON-OFF switching devicecan also be used for mixing, heterodyning, or detection, since suchdevices also exhibit second-order transfer function characteristics. Ifmore than two high frequency carrier signals are transmitted, othernonlinear transfer functions, such as 3^(rd) order and higher, areefficient for the detection process.

When an ON-OFF switching device is used as the nonlinear device, it canalso provide signal detection. In this case the output of the nonlineardevice is the desired output signal.

In another embodiment of the invention the low power receiver is adaptedto receive spread spectrum carriers. Spread spectrum techniques mayutilize frequency hopping, or direct sequence spreading, or any otherspread spectrum techniques known in the art.

Spread spectrum techniques result in spreading the transmitted energyover a wider frequency band. This can be important when governmentregulatory agencies impose limits on the maximum allowable radiatedpower limits for the high frequency carriers. In some countries, theregulatory agencies allow spread spectrum signals to be transmitted atpower levels higher than signals that do not employ spread spectrumtechniques. It may be beneficial to use spread spectrum techniques totransmit the signals at higher power levels so as to overcome radiofrequency propagation losses. Transmitting at higher power levels toovercome radio frequency propagation losses improves the performance andreliability of the communications link. Another benefit is that thesignals are more resistant to transmission impairments such as multipathfading and interference.

There are two fundamental methods for producing spread spectrum signals:(1) direct-sequence spread spectrum, and (2) frequency hopping spreadspectrum.

Direct-sequence spread-spectrum transmissions multiply the data beingtransmitted by a “pseudorandom noise” signal. This pseudorandom noisesignal (PN) is a sequence of 1 and −1 values, called “chips”, operatingat a rate much higher than that of the original data signal. Multiplyingthe data to be transmitted by a high-rate PN sequence causes the energyof the original signal to be spread over a much wider frequency band.Since the sequence of the chips produced by the transmitter is known bythe spread spectrum receiver, the receiver can use the same PN sequence,to counteract the effect of the pseudorandom noise sequence on thereceived signal, in order to reconstruct the original information.

In one preferred embodiment of the invention, direct-sequence spreadspectrum techniques may be employed if the two transmitted highfrequency carriers use the same PN chip sequence at the same period intime. When spreading in this manner, the instantaneous intermediatefrequency, or baseband frequency, recovered by the low-power receiverwill be the same as if spreading had not been employed. The low-powerreceiver will recover the transmitted signal information.

In frequency hopping spread spectrum systems, the transmitter's carrierfrequency hops over a band of frequencies using a specific hop sequence.The associated spread spectrum receiver also tunes over the samefrequency band using an identical frequency hop sequence at the sameperiod in time. In this way, the transmitter and receiver are alwaysoperating on the same RF frequency, or channel, and hence the receiveris able to recover the transmitted information.

In one preferred embodiment of the invention, spread spectrum frequencyhopping techniques may be employed if the two transmitted high frequencycarriers frequency hop using the same hop sequence and hop at the same,or nearly same, instant in time. It is desirable that the two highfrequency carriers maintain their desired frequency differencerelationship while hopping during the spectrum spreading sequence. Whenfrequency hopping in this manner, the intermediate frequency of the twohigh frequency carriers will maintain the desired relationship and itwill be possible to recover the desired control signal or transmitteddata with the low-power receiver.

In still another embodiment of the invention, the transmitter is adaptedto transmit at least two pairs of high frequency carriers, and the firstpair of high frequency carriers has a difference in frequency that isequal to the difference in frequency of the second pair of highfrequency carriers. Both high frequency carrier pairs will produce thesame intermediate frequency, or beat frequency, in the receiver, i.e.Fout=|F2−F1|=|F4−F3|. The two pairs of transmitted high frequencycarriers convey the same signal information, and hence they areredundant. Thus the desired signal information is radiated twice andreceived twice since the superposition of the first pair of highfrequency carriers is redundant with the superposition of the secondpair of high frequency carriers. Transmitting a second pair of signals,that contain redundant information, is advantageous in communicationslinks that experience multipath fading. If one of the pair of carriersexperiences destructive interference due to multipath fading, it isunlikely the second pair will be affected since their wavelengths willbe different; and multipath fading is dependent upon the carrierwavelength. As a consequence, the receiver will be able to recover thetransmitted information and maintain a reliable communications link evenin the presence of severe multipath fading. Therefore the communicationlink established with the proposed low power receiver is more resistantto link failure due to multipath fading. This can be attributed to theredundancy of the desired signal in two pairs of high frequencycarriers.

Description of the Transmitter

According the invention, the signals required for the aforementioned lowpower receiver are fulfilled by a transmitter comprising at least oneantenna, and at least one high frequency amplifier, and at least onesignal source providing at least two high frequency carrier signals. Thetransmitter is adapted to transmit a first high frequency carrier and asecond high frequency carrier signal simultaneously. The high frequencycarrier signals are adapted to be down-converted to an intermediatefrequency signal or baseband frequency signal in the low power receiver.

According to the invention, the transmitter produces a pair of radiatedhigh frequency carrier signals such that the signals contain anintermediate frequency signal, or a baseband signal, when superimposed.The intermediate frequency, or baseband frequency, Fout, is thedifference frequency of the high frequency carriers F1 and F2, i.e.Fout=|F2−F1|. This has the advantage that when the two high frequencycarriers are received by the low power receiver, the intermediatefrequency signal is already contained in the superposition of the twohigh frequency carriers, wherein the otherwise needed local oscillatorsignal required in a super-heterodyne receiver for obtaining theintermediate frequency signal is replaced by one of the high frequencycarriers. Thus a simplified receiver design can be utilized, whichcontains less high frequency electronic circuitry, and thus can bedesigned to consume very low power. The transmitter disclosed in theinvention provides a signal to replace the local oscillator signal thatis required in receivers based upon the super-heterodyne and homodyneprinciples of operation.

It should be noted that the amplitude of the two high frequency carriersmay be equal or may be different, depending on the transmitter and/orreceiver design.

The transmitter can comprise one radio frequency amplifier and antennafor all high frequency carriers, or one radio frequency amplifier andantenna for each radiated high frequency carrier. Alternatively, thetransmitter may employ one radio frequency amplifier for each separatehigh frequency carrier, and combine the outputs of the amplifiers intoone or more antennas.

It may be desirable to use a separate amplifier for each high frequencycarrier to simplify the design of the transmitter. By using a separateamplifier for each high frequency carrier, the amplifiers' linearityrequirements are less critical, and the amplifier may be less costly tomanufacture. High frequency amplifiers that simultaneously amplifymultiple carrier signals require a high degree of linearity to preventintermodulation and distortion of the signals. In addition, highfrequency amplifiers that only amplify one high frequency carrier at atime can be designed to be more power-efficient than amplifiers designedto simultaneously amplify more than one carrier signal. Amplifiersoperating with high power efficiency are smaller, lighter in weight,easier to cool, and cost less to manufacture than high-linearityamplifiers.

In one embodiment of the transmitter according to the invention, thetransmitter does not include a modulation unit. The transmitter isadapted to only transmit two high frequency continuous wave (CW) carriersignals. Either one or both CW carrier signals may be enabled ordisabled as desired. The presence, or absence, of one, or both, CWcarrier signals can be used as a command signal to the low powerreceiver

In another embodiment of the transmitter according to the invention, thetransmitter can transmit a signal containing information to thereceiver. The transmitter comprises at least one modulation unit that isadapted to modulate at least one high frequency carrier signal with adata signal. The data signal may be either a digital or an analogsignal. The data signal can either be a data stream, or repeatedinformation like an identification code. The carriers may be amplitudemodulated (AM), ON-Off Key modulated (OOK), frequency modulated (FM),phase modulated (PM), quadrature-amplitude modulated (QAM), or anycombination thereof.

In another embodiment of the transmitter according to the invention,both high frequency carriers may be modulated. The carriers may beamplitude modulated (AM), ON-Off Key modulated (OOK), frequencymodulated (FM), phase modulated (PM), quadrature-amplitude modulated(QAM), or any combination thereof. The data signal may be either adigital or an analog signal. The data signal can either be a datastream, or repeated information like an identification code. Themodulation technique can add up to a combined modulation in theintermediate frequency signal as further described below.

In one embodiment of the invention the transmitter is adapted toquadrature-amplitude modulate the data signal on at least one of thehigh frequency carriers.

If the data signal is quadrature-amplitude modulated on one highfrequency carrier, each transmitted symbol can convey more than one databit. There are 8 QAM, 16 QAM, up to 256 QAM methods known in the art.

In another embodiment of the invention, one high frequency carrier maybe amplitude modulated, and the second high frequency carrier may bephase modulated. The resulting intermediate frequency signal, which isthe superposition of the two high frequency carriers, is then a QAMsignal or pseudo-QAM signal.

The digital data may be pre-processed, or filtered, before it ismodulated onto the high frequency carriers. Known pre-processing stepsinclude analog filtering, and/or digital filtering, to reduce thespectral occupancy of the signal information. Filter types may includeGaussian, Butterworth, Root-raised Cosine, etc. Examples for digitalmodulation techniques that may be used include, but are not limited toamplitude modulation (AM), On-Off Keying (OOK), frequency-shift keying(FSK), minimum-shift keying (MSK), Gaussian-frequency-shift keying(GFSK), Gaussian-minimum-shift keying (GMSK), phase-shift keying (PSK),orthogonal frequency division multiplexing (OFDM), which is also a QAMtechnique, quadrature phase-shift keying (QPSK), etc.

In still another embodiment of the invention, the transmitter is adaptedto transmit at least two pairs of high frequency carriers, and the firstpair of high frequency carriers has a difference in frequency that isequal to the difference in frequency of the second pair of highfrequency carriers. Both high frequency carrier pairs will produce thesame intermediate or beat frequency, in the receiver, i.e.Fout=|F2−F1|=|F4−F3|. The two pairs of transmitted high frequencycarriers may be modulated using the methods described earlier, andconvey the same signal information, and hence they are redundant. Thusthe desired signal information is radiated twice and received twicesince the superposition of the first pair of high frequency carriers isredundant with the superposition of the second pair of high frequencycarriers.

Transmitting a second pair of signals, that contain redundantinformation, is advantageous in communications links that experiencemultipath fading. If one of the pairs of carriers were to experiencedestructive interference due to multipath fading, it is unlikely thesecond pair will be affected since their wavelengths will be different;and multipath fading is dependent upon the carrier wavelength. As aconsequence, the receiver will be able to recover the transmittedinformation and maintain a reliable communications link even in thepresence of severe multipath fading. Therefore the communication linkestablished with the low power receiver is more resistant tocommunication link failure caused by multipath fading. This can beattributed to the redundancy of the desired signal in two pairs of highfrequency carriers.

In one embodiment of the invention, the transmitter is adapted totransmit the data using spread spectrum techniques. This has theadvantage that the radiated electromagnetic energy is spread over abroader frequency band, and therefore the average power spectral densityof the radiated energy is reduced. This is especially important wherestate regulations on radiated power limit the maximum legal transmitterpower level. Transmitters employing spread spectrum techniques are oftenallowed to transmit at higher power levels than those that do not usespread spectrum. Transmitting at higher power levels is advantageous forincreasing the operating range of the communications link, or increasingthe link reliability, or both.

Another benefit of using spread spectrum techniques is that the signalis harder to detect, and therefore more secure for sensitive data. Insome cases the spread spectrum signal is not detectible unless thereceiver knows the spread spectrum technique a priori.

Description of a Low Power Transmitter

A further aspect of the invention is a low power transmitter comprisingan antenna and a nonlinear device coupled to the antenna. The nonlineardevice comprises a signal input for receiving signals having at leasttwo different frequencies F1 and F2 and a signal output. The nonlineardevice is adapted to generate at least signals having two furtherfrequencies F₂₁ and F₁₂ from received signals F1 and F2 and to couplethe signals having said at least two further frequencies F₂₁ and F₁₂back to the antenna. Thus the majority of the circuitry in the low powertransceiver is passive and does not require a source of DC power tooperate. The only DC power required is that necessary to modulatesignals F₂₁ and F₁₂ (or, in general terms: F_(mnp)) with the data signalto be transmitted. Since this function consumes very little power, thelow power transmitter consumes very little power from the implant'sbattery when it is transmitting. This is a considerable advantage overconventional transmitter architectures.

However, the nonlinear device of the transmitter may be connected to anamplifier or may be part of an amplifier in order to increase thetransmitter's range.

According to preferred embodiments, the low power transmitter may beadapted to perform on-off-keying (OOK) to transmit amplitude modulated(AM) signals or to perform frequency shift keying (FSK) as pointed outbelow.

Description of a Low Power Transceiver

The basic elements comprising the invention are: a non-implantedtransceiver as described above that simultaneously transmits multiple RFsignals, and an implantable low-power transceiver that employs nonlinearcircuitry to simplify the transceiver's architecture and minimize powerrequirements.

The non-implanted transceiver utilizes conventional RF circuitry andtechniques to implement the necessary receiver and transmitterfunctions, but it is unique in that it simultaneously transmits multipleRF carrier signals that may be modulated by a data signal. Thistransceiver, which may be part of a physician's programmer, or bedsidemonitor, is typically powered by the AC mains, and hence its size andpower consumption are not as critical as the low-power transceiver thatis implanted in a patient.

The implanted transceiver is unique in that it utilizes the nonlinearbehaviour of circuit elements to simplify its architecture. Simplifyingthe implanted transceiver's architecture can make the transceiver verypower-efficient, physically small, and highly reliable. These attributesare extremely desirable in medical implants such as pacemakers and ICDs.In addition to being a low power receiver, the low power transceiver canalso act as a low power transmitter.

A low power transceiver according to a preferred embodiment of theinvention comprises a low power transmitter according to the inventionand a low power receiver according to the invention.

Preferably both, the low power transmitter and the low power transceivereach comprise a nonlinear device that is connected to a single antennaby means of a signal combiner/splitter for connecting said receiver'snonlinear device or said transmitter's nonlinear device to said antenna.

Description of the Modulation and Signal

The following modulation techniques apply to the communication methodaccording to the invention.

-   -   1. None of the simultaneously transmitted high frequency        carriers may be modulated. The carriers, (F1 and F2), may be        high frequency continuous wave (CW) signals. In this case, the        desired signal is the frequency difference signal, i.e.        Fout=|F2−F1|.    -   2. It should be noted that either one high frequency carrier, or        more high frequency carriers, may be modulated. The carrier, or        carriers, may utilize amplitude modulation (AM), On-Off Key        modulation (OOK), frequency modulation (FM), phase modulation        (PM), quadrature-amplitude modulation (QAM), or any combination        of AM, OOK, FM, PM, or QAM modulation techniques.    -   3. This data signal may be a data stream or a single repeated        sequence such as an identification code.    -   4. The data signal can either be an analog signal or a digital        signal.    -   5. The high frequency carrier, or carriers, may be modulated        with a cyclic signal, which can be frequency coded to address        one specific receiver. This would be one bit of information        —either the frequency of the cyclic signal matches a        predetermined frequency or not.    -   6. In one embodiment of the communication method according to        the invention, one high frequency carrier is amplitude modulated        with a first part of the signal information, and a second high        frequency carrier is phase modulated with a second part of the        signal information, wherein the resulting intermediate frequency        signal, which is the superposition of the two high frequency        carriers, is quadrature-amplitude modulated and contains the        full signal information.    -   7. The high frequency carrier signals may also employ spread        spectrum techniques such as direct sequence spread spectrum or        frequency hopping techniques.

The invention has been described in different embodiments. It should benoted that the features of the different embodiments can be combined, ifthey do not exclude each other, or if it is otherwise specified.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing, and other objects, advantages, and novel features of thepresent invention can be understood and appreciated by reference to thefollowing detailed description of the invention, taken in conjunctionthe accompanying drawings, in which:

FIG. 1 shows schematically a frequency spectrum of a transmitter withtwo carriers.

FIG. 2 depicts schematically a possible simplified design of atransmitter 200 according to one embodiment of the invention.

FIG. 3 shows an embodiment of a simplified schematic design of areceiver 300 according to the invention.

FIG. 4 shows schematically a transmitter frequency spectrum of anotherembodiment of the invention.

FIG. 5 shows one possible embodiment of a nonlinear electronic circuitthat can be used in the low-power receiver to generate an intermediatefrequency signal, or a baseband signal, from the superposition andmixing of two or more high frequency carrier signals.

FIG. 6 shows a wireless communications system designed for a low-powertransceiver.

FIG. 7 shows a signal spectrum for two signals (F1 and F2) applied to adevice with a nonlinear transfer function.

FIG. 8 shows a signal spectrum for three signals (F1, F2, and F3), withequal frequency separation, applied to a device with a nonlineartransfer function.

FIG. 9 shows a signal spectrum for three signals (F1, F2, and F3), withunequal frequency separation, applied to a device with a nonlineartransfer function.

FIG. 10 shows an Implantable low-power transceiver with a TX amplifierto increase the operating range.

DETAILED DESCRIPTION OF THE INVENTION

The following description is of the best mode presently contemplated forcarrying out the invention. This description is not to be taken in alimiting sense, but is made merely for the purpose of describing thegeneral principles of the invention. The scope of the invention shouldbe determined with reference to the claims.

FIG. 1 shows, schematically, the frequency spectrum of a transmitterwith two high frequency carriers at different frequencies F1 and F2. Thehigh frequency carriers may not have modulation sidebands (i.e. they maybe only CW carriers), or one, or both carriers may have modulationsidebands 106. None, one, or all high frequency carriers may bemodulated. In the following the high frequency carriers may be referredto simply by carriers. The carriers of two frequencies mix with eachother and produce new signals at the frequency F1+F2 and at deltaF=|F2−F1|. Delta F can be used for a low power receiver design, becausealthough F1 and F2 may be very high, delta F can be very low. The signalat frequency delta F is called intermediate frequency signal. If delta Fis very small, it may also be referred to as a baseband signal. Becauseof this relationship between two carriers, or more than two carriers,the design of the receiver can be simplified as described further below.

FIG. 2 schematically depicts a possible simplified design of atransmitter 200 according to one embodiment of the invention. Thetransmitter is adapted to simultaneously transmit two different carriersat a first high frequency F1, and a second high frequency F2, which isdifferent from the first frequency. In this embodiment the transmitter200 comprises a control and data unit 202 to provide referencefrequencies, and is optionally adapted to perform analog and/or digitalbaseband modulation. The transmitter further comprises two highfrequency sources 204 and 206, which are connected to the control anddata unit 202 at their inputs. These high frequency sources generate thetwo carriers at different frequencies. The first radio frequency source204 generates the first carrier at the frequency F1 and the second highfrequency source 206 generates the second carrier at the frequency F2.The high frequency sources 204, 206 are connected at their respectiveoutput to a combiner 208, which linearly combines the two high frequencysignals. The combiner 208 is connected to an antenna 210, which radiatesthe high frequency signals over the air. The high frequency sources 204and 206 may utilize separate high frequency amplifiers to increase thesignal power of the carriers prior to being combined. Optionally, thecombiner 208 may also include high frequency amplifiers to furtherincrease the power level of the signals to be transmitted.

In another embodiment of the antenna configuration, the combiner stage208 is not used, and the high frequency sources 204, and 206, areindividually connected to separate antennas.

In one embodiment, one high frequency source 204 is adapted to modulatea baseband signal to the high frequency carrier and the other highfrequency source 206 is adapted to produce a continuous wave (CW) highfrequency signal, which acts as a reference signal for a receiver. Inanother embodiment both high frequency sources 204 and 206 are bothadapted to modulate baseband signals onto the two carriers of differenthigh frequencies.

In one embodiment of the invention one high frequency carrier isamplitude modulated and the other high frequency carrier is phasemodulated. The resulting intermediate frequency produced in the lowpower receiver, which is the superposition of the two carriers, is aquadrature-amplitude modulated (QAM) signal.

In another embodiment of the invention, more than one pair of highfrequency sources are provided. The sources may be combined andtransmitted using one antenna, or the sources may not be combined, andeach source will transmit its signal from a separate antenna, i.e. oneantenna for each source. Transmitting more than one pair of signals canbe used to improve the reliability of the communications link byreducing the susceptibility to signal fading caused by multipathinterference.

In one embodiment of the invention the control and data unit is aprogrammable controller, an application specific integrated circuit(ASIC), or a digital signal processor (DSP), or a combination thereof.In another embodiment the high frequency sources are numericallycontrolled oscillators (NCO), crystal oscillators, or phase-lockedfrequency synthesizers.

FIG. 3 shows an embodiment of a simplified schematic design of areceiver 300 according to the invention. In this embodiment the receivercomprises an antenna 302, which is connected to a first band-pass filter304 being connected to a nonlinear or switching device 306. Thenonlinear or switching device is connected to an intermediate frequencybandpass filter, or a baseband filter, and an amplification unit 308.This unit is connected to a level detector unit 310, and to a datadetection unit 314.

The antenna is tuned to receive two high frequency carriers atfrequencies F1 and F2, which are of similar frequencies. The two highfrequency carriers are fed to the first band-pass filter 304. Thisbandpass filter has a narrow pass band around a middle frequency betweenF1 and F2. The filter is designed to allow both high frequency carriersto pass through the filter with little attenuation, while attenuatingunwanted signals outside the filter's pass band.

The filtered high frequency signals are fed to the nonlinear orswitching device 306. The superposition and mixing, beating, orheterodyning of the two high frequency carriers in the nonlinear, orswitching device, 306 results in two additional frequencies, Fsum=F1+F2,and Fout=|F2−F1|. The desired intermediate frequency signal, or basebandsignal, is the signal at frequency Fout=|F2−F1|. The IF signal, orbaseband signal, is fed to the IF filter, or baseband filter, andamplification unit 308. The IF or baseband filter passes the desiredsignal with very little attenuation, and suppresses the residual highfrequency components at F1 and F2, as well as at Fsum.

The filtered and amplified IF or baseband signal is fed to the leveldetector unit 310, which detects if the signal exceeds a predeterminedlevel. If the predetermined level is exceeded the level detector unitgenerates a wake up signal 312 to activate other parts of the device,e.g. a more power consuming communication device and/or a correspondingprocessor can be activated. The signal is possibly also fed to a datadetection unit 314. In one embodiment of the invention the datadetection unit may detect and output a data signal 316. Alternatively,the output of the data detection unit may also be used as a wakeupsignal.

In one embodiment the data signal is an identification code for thedevice in which the receiver is installed. Thus only the receiver withthe matching identification code may wake up when a transmittertransmits the corresponding identification code. In another embodimentof the invention, the transmitter may transmit a broadcast message toall receivers within reception range, and all devices will respond tothe transmitted message. In yet another embodiment of the invention, thedata detection unit can feed the data from the received baseband signalto other parts of the device to allow the device to receive data from atransmitter.

FIG. 4 shows a schematic of the transmitter frequency spectrum ofanother embodiment of the invention. With this implementation of theinvention, it is possible to reduce communication link failures due tomultipath fading. Multipath fading means that radio signals can beseverely attenuated at certain locations due to destructive interferencecaused by signal reflections from nearby structures.

In this embodiment two pairs of high frequency carriers, (four signals),402 and 404, are used to transmit data. The two pairs of high frequencycarriers 402 and 404 have the same frequency difference delta F.(F4−F3)=(F2−F1)=delta F.

In the receiver, the intermediate frequency, delta F, is received due tothe above described superposition and heterodyning of each pair of highfrequency carriers. Thus on the receiver side the two high frequencycarrier pairs have the same intermediate or baseband frequency. Althoughthe receiver receives only one intermediate frequency signal, orbaseband frequency signal, it consists of two pairs of independent highfrequency signals due to superposition.

In this embodiment of the invention the high frequency carrier pairs F1,F2 and F3, F4 carry the same signal information. That means they areredundant and thus the information in the resulting intermediatefrequency signals is redundant. When the signals are received at thereceiver without reflections, or multipath interference from nearbystructures, the resultant received intermediate frequency signal is asuperposition and heterodyning of the two pairs of high frequencycarriers. When multipath fading occurs on one high frequency carrierpair, the receiver will still receive the other high frequency carrierpair, and can receive the signal information on its respectiveintermediate frequency signal. It is very unlikely that multipath fadingoccurs on the two high frequency pairs at the same receiver locationsince the high frequency carrier pairs are in a different frequencybands and therefore their wavelengths are different. Although thecommunication link may fail with the F1 and F2 pair, the link remainsestablished by F3 and F4 pair. The same situation is true if the F3 andF4 pair experience multipath interference. The receiver will likelyreceive the desired signal information transmitted by the F1 and F2 pairof high frequency carrier signals.

FIG. 5 shows one possible embodiment of a nonlinear electronic circuit500 that can be used in the low-power receiver to generate an IFfrequency signal, or a baseband frequency signal, by the superpositionand mixing of two or more high frequency carrier signals. The input tothe nonlinear stage is labeled RF Input, and it receives the highfrequency carriers from the antenna and band-pass filter (not shown inthis diagram). A balun transformer 501 converts a single-ended signal toa differential signal, and provides an impedance transformation toincrease the voltage level of the received signal. The differentialsignal comprised of the high frequency carriers is coupled to diodes505, 506, 508 and 509. These diodes are configured to operate as afull-wave rectifier, and develop an output signal across load resistor507. Capacitors 502 and 504 serve to block direct current flow, and alsopresent a high impedance to block the generated IF or baseband outputsignal, but allow the high frequency carrier signals to couple from thebalun transformer to the diodes. The nonlinear behavior of the full waverectifier results in the desired intermediate frequency (IF) signal, orbaseband signal, being developed across load resistor 507. Inductors 503and 510 present a high impedance to the high frequency carriers, butallow the desired IF output signal, or baseband output signal, to passwith minimal attenuation. Inductor 503 also provides a ground returnpath for the IF or baseband output signal. The desired IF output signal,or baseband signal, is available at the terminals labeled IF or BasebandOut. Capacitor 511, in conjunction with inductor 503 and inductor 510,operates as a low-pass filter to attenuate the high frequency carriers,and the unwanted mixing product resulting from the sum of the twocarriers (Fsum). The cutoff frequency of this output filter section canbe adjusted to accommodate the desired output frequency of the IF, orbaseband output signal, by adjusting the component values accordingly.

It should be noted that this is only one of many possible methods forimplementing a nonlinear circuit to develop the desired IF signal orbaseband signal from the high frequency carrier input signals. Forexample, the input balun transformer could be replaced with aninductor-capacitor network to provide impedance transformation and/orconvert a single-ended signal to a differential signal. Also, if thenonlinear circuit was coupled to the output terminals of a dipoleantenna, the balun stage would not be needed since a dipole antennaproduces a signal output that is already in a differential form. It isalso possible to implement the nonlinear circuit using a single diodesuch that single-ended to differential conversion is not required andthe need for a balun can be eliminated. Furthermore, there aretechniques know in the present art for producing nonlinear operationusing other diode configurations, such as “half bridge” configuration,and the “ring quads” configuration as found in conventionaldouble-balanced mixers. Zero-bias Schottky diodes, with very low barrierpotential, result in good signal sensitivity and conversion efficiency,however, other diode types can be used and will result in varyingdegrees of circuit performance. Tunnel diodes and Back diodes and alsobe used, but they are significantly more expensive than Schottky diodes.

In FIG. 6 a block diagram of the wireless communications system designedfor a low-power transceiver is shown. Although this invention can beused in numerous non-medical applications, the following disclosuredescribes how the system could be used to implement a wireless telemetrylink between an implanted medical device, such as a pacemaker, and anon-implanted device, such as a physician's programmer.

The elements contained within the dashed lines on the left side of FIG.6 are functional blocks of an RF transceiver 600 that could be includedin the non-implanted device. Since this transceiver is not implanted,and power consumption and size are not overly critical, it can beimplemented using conventional RF transceiver architectures andtechniques. The elements contained within the dashed lines on the rightside of FIG. 6 form a low-power transceiver 650 that could beincorporated in a medical implant. Since low power consumption and smallphysical size are critical design requirements for medical implants, thedesign of the wireless communications system stated herein is focused onachieving these attributes in the implanted transceiver.

FIG. 6 also shows RF communications signals 602, 604 between thenon-implanted transceiver (NIT) 600 and the implanted low-powertransceiver (LPT) 650. The carrier signal transmitted by the LPT 650 tothe NIT 600 is shown as the dashed line labeled Fmnp 602. The carriersignals transmitted by the NIT 600 to the LPT 650 are shown as dashedlines labeled F1, F2, . . . Fn 604. Multiple dashed lines are shown forthese signals to indicate that they are transmitted simultaneously fromthe NIT 600 to the LPT 650.

To gain an understanding of how the invention operates, the followingdisclosure will describe the theory of operation of the LPT 650 and NIT600 separately, operating in both transmit and receive modes.

Theory of Operation—LPT Transmitter

The following description details the operation of the LPT 650 operatingas a transmitter, i.e. transmitting an RF carrier that may be modulatedwith patient data, or implant status information, or both, to the NIT600.

The principle mechanism governing the operation of the LPT 650 whenoperating as a transmitter is that when multiple RF carrier signals 604,at frequencies F1, F2, F3, . . . etc., are simultaneously applied to anelectronic circuit with a strong nonlinear transfer function, additionalRF signals 602 are created at frequencies Fmnp=|±mF1±nF2±pF3 . . . |where m, n, p, etc. are integers (theoretically, m, n, p . . . rangefrom 0 to infinity).

In the special case where two signals 702, (F1 and F2), aresimultaneously applied to a nonlinear circuit 700, several frequencies704 are generated as shown in FIG. 7. Of particular interest are thesignals created at frequencies F₁₂=|F1−2F2| and F₂₁=|2F1−F2|. Note thatF₁₂ and F₂₁ are at frequencies above and below F2 and F1 by ΔF=(F2−F1).

By appropriately choosing frequencies F1 and F2, the signal componentsF₂, and F₁₂ can be advantageously located in the radio spectrum suchthat they can be easily distinguished from F1 and F2, yet close enoughto fall within the same RF band as F1 and F2. Because the frequencies ofF₂₁ and F₁₂ are relatively close to F1 and F2, they will couple backinto the antenna and radiate just as if they were created by separateoscillator circuitry or signal sources in the nonlinear device 700.

The LPT 650 operates as a transmitter using the aforementioned techniqueof generating new signals at F₂₁ and F₁₂. The two signals F1 and F2 areradiated by the NIT 600 and received by the LPT antenna (A2) 652 asshown in FIG. 6. The signals are then coupled from the LPT's antenna652, via a linear Signal Combiner (SC2) 654, to Switch (SW) 656 and theTX Nonlinear Device (TX-NLD) 658. The Signal Combiner (SC2) 654 can bereplaced by a switch to connect either the low-power receiver 662 or thelow-power transmitter (sections 656, 658 and 660) to the antenna (A2)652. The section labeled TX-NLD 658 is a nonlinear device that generatesF₂₁ and F₁₂ from received signals F1 and F2. As outlined above, F₂₁ andF₁₂ are coupled back to the antenna A2 652, (via Switch SW 656 andSignal Combiner SC2 654), and are radiated out of the LPT 650. AnRF-Receiver RX 610 tuned to F₂₁ or F₁₂ can receive these signals. Byusing Switch (SW) 656 to connect and disconnect the TX Nonlinear Device(TX-NLD) 658 to the LPT's antenna 652, in accordance with the state of abinary digital data signal, the LPT 650 can transmit data to the NIT600. When Switch SW 656 is in the closed state, signals at F₂₁ and F₁₂are generated and subsequently transmitted. When Switch SW 656 is in theopen state, no signals are generated (or transmitted) at F₂₁ and F₁₂.The presence/absence, of a signal at F₂₁ (or F₁₂) effectively implementsON-OFF Keying of an RF carrier that is modulated by digital data fromthe LPT 650.

As an alternative to controlling the Switch (SW) 656 to produce OOKmodulation, the data signal could also modulate, or alter, thecharacteristics of the TX-NLD's 658 nonlinear transfer function. Thisapproach could further simplify the LPT 650 circuitry by eliminatingSwitch SW 656. By modulating, or altering, the TX-NLD's 658 nonlinearityin accordance with the data signal, amplitude modulation (AM), and/orOOK modulation, can be produced. In the LPT 650, the section labeledModulator (MOD) 660 would also provide any necessary level shifting ofthe data signal to a voltage level appropriate for controlling Switch(SW) 656 or the TX Nonlinear Device (TX-NLD) 658.

It should be noted that the LPT 650 could be implemented by transmittingmore than two signals. For example, FIG. 8 shows how three signals (F1,F2, and F3) 802 can be used to transmit a signal from the LPT 650 to theNIT 600.

This system, using three signals 802, operates using the same principlesas the system of FIG. 7 previously described. The only significantdifference is that the signals transmitted by the LTP 650 are the resultof different mixing products arising from the presence of the additionalsignal, F3, combining with F1 and F2 804 in the TX-NLD 800.

Using three RF carriers, (F1, F2, and F3) 802, the LPT 650 can transmitsignals at: F_(mnp)=|±mF1±nF2±pF3|804. If the three carrier frequenciesare equally spaced, i.e. ΔF=(F2−F1)=(F3−F2), then the nonlinear devicewill produce useful transmit signals for coefficient sets [m=+1, n=+1,p=−1] and [m=−1, n=+1, p=+1]. FIG. 8 shows the signal spectrum for thissituation, with the signals at F⁺¹⁺¹⁻=|+F1+F2−F3| andF⁻¹⁺¹⁺¹=|−F1+F2+F3| suitable for transmission of data from the LPT 650to the RF-receiver RX 610 in the NIT.

It should be noted that the majority of the circuitry in the LPT 650 ispassive and does not require a source of DC power to operate. The onlyDC power required is that necessary to modulate signals F₂₁ and F₁₂ (or,in general terms: F_(mnp)) with the data signal to be transmitted. Sincethis function consumes very little power, the LPT 650 consumes verylittle power from the implant's battery when it is transmitting. This isa considerable advantage over conventional transmitter architectures.

Transmitting FSK Signals:

The above description detailed the operation of the LPT's transmitterbased upon utilizing two signals, (or three equally spaced signals), toproduce an OOK or AM modulated signal. By employing three signals withasymmetrical frequency spacing, the LPT's transmitter can transmit anFSK modulated signal. Although this requires a third signal source inthe NIT 600, FSK-based telemetry systems offer improved link margin andlink reliability over AM and OOK systems when the communications channelis impaired by impulse noise.

FIG. 9 shows the signal spectrum for three carriers 902, withasymmetrical frequency spacing, transmitted from the NIT 600 to the LPT650. For carriers F1, F2, and F3: ΔF1=(F2−F1), ΔF2=(F3−F2), and ΔF1≠ΔF2.The signals of interest that are transmitted by the LPT are the twosignals at F⁺²⁻¹⁺⁰ 904 and F⁺⁰⁺²⁻¹ 906.

Note that F⁺³⁻¹⁺⁰=|+2F1−F2| 904 and F⁺⁰⁺²⁻¹=|+2F2−F3| 906 are very closein frequency, and F⁺²⁻¹⁺⁰ 904 is the result of the mixing of only F1 andF2, while F⁺⁰⁺²⁻¹ 906 is the result of the mixing of only the F2 and F3signals. With three signals 902 transmitted by the NIT 600, the LPT 650can transmit an FSK signal, comprised of the two signals F⁺²⁻¹⁺⁰ andF⁺⁰⁺²⁻¹, by selectively filtering the frequency pair F1 and F2, or F2and F3, and applying only the desired selected frequency pair to the TXNonlinear Device (TX-NLD) 900. This frequency pair selection can beimplemented using bandpass filters to select the desired two signals, orusing a notch-filter to remove the unwanted signal (F1 or F3) asnecessary. The frequency selection and filtering can be included as apart of Switch SW 656, as indicated in the LPT 650 section of FIG. 6.

The following is an example of one possible method for implementing FSKdata transmission from the LPT 650 using the aforementioned technique.Consider the case where the NIT 600 transmits three signals to the LPT650 that have the following frequencies:

-   F1=914.00 MHz-   F2=915.00 MHz-   F3=916.99 MHz

In the LPT 650, bandpass filters, or notch filters, are used toselectively apply the frequency pairs of either F1 and F2, or F2 and F3,to the nonlinear device TX-NLD 900. If the F1 and F2 pair are assignedto represent a logic state of “0”, and the F2 and F3 pair represent thelogic state of “1” (the two states representing a binary data bit), thenthe LPT will transmit a signal at F⁺²⁻¹⁺⁰=|+2F1−F2|=913.00 MHz 904 for alogic “0”, and it will transmit a signal at F⁺⁰⁺²⁻¹=+2F2−F3|=913.01 MHz906 for a logic “1”. Because these two frequencies represent twodistinct logic states, the LPT 650 has effectively transmitted an FSKmodulated signal. Also note that the FSK frequency separation, i.e.ΔF_(pp)=F⁺⁰⁺²⁻¹−F⁺²⁻¹⁺⁰, is determined by the frequency relationshipbetween F1, F2, and F3. In this example, a narrow frequency separationwas used, (10 kHz), to indicate that a narrowband FSK signal could betransmitted by the LPT 650. It is advantageous to utilize narrowband FSKdata transmission in medical telemetry applications because theamplitude of the signals at F₊2−1+0 904 and F⁺⁰⁺²⁻¹ 906 are very weak,and narrowband FSK receivers with excellent sensitivity are readilyavailable for the NIT 600. It should be noted that the frequencies usedin this example were chosen only to demonstrate the principle ofoperation of the invention, and other frequencies can be used toimplement the invention in the same, or other, radio frequency band(s).

Depending upon the frequencies chosen for the carriers F1, F2, and F3;high-Q crystal, ceramic, SAW, or MEMS resonators, could be employed toimplement the bandpass or notch filters employed in the LPT FSKtransmitter.

Still another approach, when transmitting more than two carriers, is toselectively switch between two different nonlinear devices connected tothe antenna of the LPT 650. Using different nonlinearities to generatedifferent mixing products will also result in an FSK-modulated signal.

Extending the Operating Range With an Implant Transmitter Amplifier:

In some applications it may be desirable to extend the operating rangebetween the LPT 1000 and NIT 600. Since the signal transmitted by theLPT 1000 is at a very low level, it may be advantageous to boost itslevel to increase the signal-to-noise ratio at the NIT's receiver 610.This can be accomplished, as shown in FIG. 10, by coupling an RFamplifier to the TX-NLD 1002 section. Alternatively, the amplifier canbe incorporated as part of the TX-NLD 1002, using its nonlinearitycharacteristics to generate the desired transmit signal.

In FIG. 10, the output of amplifier (TX AMP) 1004 is shown coupled backto Antenna A2 1006 via Switch (SW) 1008, but it could also be coupledback to the Antenna A2 1006 through the Signal Combiner SC2 1010. Toprevent feedback of the amplifier's 1004 output signal into theamplifier's 1004 input port, two filters (Filt1 1012 and Filt2 1014) mayalso be included in the LPT's transmitter. Filter Filt1 1012 must passthe carriers transmitted by the NIT 600 to the LPT 1000, and attenuatethe signal(s) generated by the TX-NLD 1002. Filter Filt2 1014 must passthe LTP signal(s) generated by the TX-NLD 1002, and attenuate thecarriers transmitted by the NIT 600. Alternatively, amplifier (AMP) 1004may be a tuned amplifier with a bandpass characteristic such that itprovides gain at the LPT transmitter signal frequencies. It is importantthat precautions be taken to ensure amplifier AMP 1004 does not becomeunstable and self-oscillate due to positive feedback within the loopcreated by the Filt1 1012, TX-NLD 1002, AMP 1004, and the Filt2 1014stages. Note that Filt1 1012 and Filt2 1014 are optional. Also note thatthe output of the TX Amp (AMP) 1004 could be connected to a separateantenna instead of connecting it back into antenna A2 1006.

Theory of Operation—LPT Receiver

As shown in FIG. 6, the LPT 650 also includes a low-power receiver toreceive control signals, data, or other information 604 transmitted fromthe NIT 600. The LPT's receiver can receive data from the NIT 600 whileit is simultaneously transmitting data 602 to the NIT 600, that is, itcan operate in full duplex mode; or it can receive data and othersignals from the NIT 600 when the LPT 650 is not transmitting, i.e.operating in half-duplex mode.

The signal 604 to the LPT's antenna (A2) 652 is coupled to the RXNonlinear Device (RX-NLD) 662 via the linear Signal Combiner (SC2) 654.The LPT's receiver utilizes multiple carriers simultaneously transmittedby the NIT 600 in order to simplify its architecture and minimize itspower consumption. The theory of operation of the LPT's receiver is aspreviously described with respect to the FIGS. 3 and 4.

Theory of Operation—NIT Transmitter

The following description details the operation of the NIT 600 operatingas a transmitter, i.e. transmitting signals 604 from the NIT 600, whichmay contain information such as control signals and/or data 604, to theLPT 650.

As shown in FIG. 6, the transmitter section of the NIT 600, labeled RFTransmitter (TX) 612, is comprised of multiple signal sources F1, F2, .. . Fn. The signal sources may be phase locked to a Frequency ReferenceSignal (Fref) 618 that may also be provided to the receiver section ofthe NIT 600 as well. The theory of operation of the NIT's transmitter612 is exactly as previously described above with respect to the FIGS. 1and 2. To avoid repeating the details of the low-power receiver'soperation, please refer to previous description.

Because this transmitter 612 is not implanted, and it is typicallypowered from the AC mains, low-power, and high-efficiency operation arenot critical design requirements. The transmitter 612 may be implementedusing conventional transmitter architectures and techniques. Also, notethat the outputs of the multiple signal sources may be combined into asingle antenna 614 using a linear Signal Combiner (SC1) 616 as shown inFIG. 6, or multiple antennas may be employed.

Theory of Operation—NIT Receiver

The following description details the operation of the NIT 600 operatingas a receiver, i.e. receiving signals 602 from the LPT 650 that maycontain information such as patient medical data, and/or implant status.

The Radio Receiver, (labeled RFReceiver RX 610 in FIG. 6), recovers thedata transmitted by the low-power transceiver LPT 650. The receiver 610may tune to any one of the multiple signals generated by the LPT'stransmitter at F_(mnp)=|±mF1±nF2±pF3± . . . |602, where m, n, p, . . .are integers ranging from 0 to infinity, and F1, F2, F3, . . . are thefrequencies of the carriers simultaneously transmitted by the NIT'stransmitter 612.

Because this receiver 610 is not implanted, and it is typically poweredfrom the AC mains, low-power, and high-efficiency operation are notcritical design requirements. The receiver 610 may be implemented usingconventional receiver architectures (such as the super-heterodyne) andtechniques. The receiver may be designed to utilize amplitude modulation(AM), On-Off Keying modulation (OOK), or frequency shift keying (FSK),or any combination thereof.

By phase-locking the receiver 610 and signal sources (F1, F2, . . . Fn)604 to the same reference oscillator 618, the receiver 610 can tuneexactly to the desired signal transmitted by the LPT 650. If desired,the receiver's antenna may be separate from the NIT's transmittingantenna(s), or it may be combined into a single unit 614 as shown inFIG. 6.

Spread Spectrum Mode of Operation

The wireless communications system designed for a low-power transceivercould employ frequency hopping spread spectrum techniques for carriersignals F1, F2, . . . etc. The method to accomplish this for the NIT 600to LPT 650 transmission link has already been outlined on pages 11 and12 of this disclosure. For the LPT-to-NIT communications link, the NIT'sreceiver (RX) 610 would be required to synchronize with the spreadspectrum frequency hopping protocol utilized by the NIT's signal sources612 to ensure the desired LPT signal 602 is recovered.

Although an exemplary embodiment of the present invention has been shownand described, it should be apparent to those of ordinary skill that anumber of changes and modifications to the invention may be made withoutdeparting from the spirit and scope of the invention. This invention canreadily be adapted to such devices by following the present teachings.All such changes, modifications and alterations should therefore berecognized as falling within the scope of the present invention.

What is claimed is:
 1. A low power receiver, comprising: an antenna; aresonant circuit; a nonlinear device, wherein said antenna forms saidresonant circuit or a part of said resonant circuit, or is directly orindirectly coupled to said resonant circuit; said antenna and saidresonant circuit tuned to receive a first high frequency carrier of apredetermined first frequency F1 and a second high frequency carrier ofa predetermined second frequency F2 simultaneously, which differ fromeach other by a predetermined intermediate or baseband frequency Fout=|F2−F1|, wherein said nonlinear device is directly or indirectlyconnected to said resonant circuit and configured to generate anintermediate frequency signal or a baseband signal, respectively, offrequency Fout, wherein said low power receiver is configured to operateaccording to a heterodyne principle to down convert a high frequencysignal to said intermediate frequency signal or said baseband signal,respectively, via a mix of said first high frequency carrier and saidsecond high frequency carrier to thus regain said intermediate frequencysignal without a local oscillator; a second nonlinear device coupled tothe antenna, said second nonlinear device comprising a signal inputconfigured to receive signals having at least two different frequenciesthat include said predetermined first frequency F1 and saidpredetermined second frequency F2 and a signal output, said secondnonlinear device configured to generate at least signals having twofurther frequencies F₂₁ and F₁₂ from received signals of saidpredetermined first frequency F1 and said predetermined second frequencyF2 and to couple the signals having said two further frequencies F₂₁ andF₁₂ back to said antenna to transmit data out of said antenna; and,wherein said low power receiver is configured to reside within animplantable device configured to be implanted in a body of a mammal andwherein said implantable medical device is a pacemaker, acardioverter/defibrillator, or a combination thereof.
 2. The low powerreceiver according to claim 1, wherein said low power receiver isconfigured to directly generate said baseband signal that representstransmitted data.
 3. The low power receiver according to claim 1,wherein said low power receiver is configured to generate saidintermediate frequency signal and to convert said intermediate frequencysignal to said baseband signal that represents transmitted data.
 4. Thelow power receiver according to claim 1, wherein said intermediatefrequency signal or said baseband signal have a frequency in a rangebetween 0 Hz and 1 MHz.
 5. The low power receiver according to claim 1,wherein said first and said second high frequency carriers are notmodulated and said low power receiver is configured to mix said firstand said second high frequency carriers within said antenna, or in aseparate mixer, or in another nonlinear device, to obtain saidintermediate frequency signal or said baseband frequency signal, whichis a data signal to be received.
 6. The low power receiver according toclaim 1, wherein said first or said second high frequency carrier is areference carrier, which is not modulated, and another high frequencycarrier is modulated with a data signal, and said low power receiver isconfigured to mix said reference carrier with said other high frequencycarrier within said antenna, or in a separate mixer, or in anothernonlinear device, to obtain said intermediate frequency signal or saidbaseband signal, which is a desired data signal to be received.
 7. Thelow power receiver according to claim 1 which comprises a firstband-pass filter, that is connected at least indirectly to said antenna,said first band-pass filter having a pass band that corresponds to saidfirst and second high frequency carriers.
 8. The low power receiveraccording to claim 7 which comprises a second band-pass filter, or alow-pass filter, that is connected to an output of said nonlineardevice, said second band-pass filter, or said low-pass filter,comprising a pass band corresponding to said intermediate frequency orsaid baseband frequency.
 9. The low power receiver according to claim 8wherein said second band-pass filter is connected to an intermediatefrequency amplifier or IF amplifier, or a baseband amplifier, and saidlow power receiver is configured to filter and to amplify saidintermediate frequency signal or baseband signal.
 10. The low powerreceiver according to claim 9, wherein said nonlinear device or saidintermediate frequency amplifier, or said baseband amplifier furthercomprises and is connected to a level detector circuit, and said lowpower receiver is configured to analyse a level of said intermediatefrequency signal, or baseband signal, or a filtered and amplifiedintermediate frequency or baseband signal, with said level detectorcircuit, and to produce a wakeup signal depending on a predeterminedlevel of said intermediate frequency or baseband signal, or saidfiltered and amplified IF or baseband signal, with said level detectorcircuit.
 11. The low power receiver according to claim 9, wherein saidnonlinear device, or said IF amplifier or said baseband amplifier, isconnected to a data detection circuit, and said low power receiver isconfigured to convert said intermediate frequency or baseband signal, orsaid filtered and amplified IF or baseband signal, with said datadetection circuit into a digital data signal and to output a digitalsignal.
 12. The low power receiver according to claim 1 furthercomprising a high frequency amplifier, which is connected to saidantenna or said resonant circuit, said high frequency amplifierconfigured to amplify said first and second high frequency carriers. 13.The low power receiver according to claim 1, wherein said low powerreceiver is configured to simultaneously receive more than two highfrequency carriers at different frequencies and to obtain an outputsignal from intermodulation products, produced by a nonlinear mix ofsaid more than two high frequency carriers.
 14. The low power receiveraccording to claim 1, wherein said nonlinear device comprises a singlediode, an array of diodes, or a switching device, or a device with astrong 2^(nd) order transfer function, or in a case of more than twocarriers, a device with a strong 3^(rd) order transfer function.
 15. Thelow power receiver according to claim 1, wherein said low power receiveris configured to receive spread spectrum carriers.
 16. The low powerreceiver according to claim 1, wherein said receiver is configured toreceive at least two pairs of redundant high frequency carriers and afirst pair of high frequency carriers having a difference in frequency,which is equal to a difference in frequency of a second pair of highfrequency carriers.
 17. The low power receiver according to claim 1,wherein said receiver is configured to receive at least two pairs ofhigh frequency carriers, and a frequency difference of said at least twopairs of high frequency carriers differ from each other, and whereinsaid receiver is further configured to generate two separateintermediate frequency signals at different frequencies from said twopairs of high frequency carriers representing two different datasignals.
 18. The low power receiver according to claim 1, wherein saidlow power receiver is configured to demodulate an amplitude-modulatedsignal, frequency modulated signal, phase modulated signal, or QAMmodulated signal.
 19. The low power receiver according to claim 1,further comprising a linear signal combiner and a switch connected inseries between said antenna and said second nonlinear device.
 20. Thelow power receiver according to claim 19, further comprising a modulatorconnected to said switch and configured to open and to close said switchto generate digitally coded signals via on-off-keying (OOK).
 21. The lowpower receiver according to claim 1, further comprising a modulatorconnected to said nonlinear device in order to alter nonlinearity of thenonlinear device in accordance with a digital data signal to generate anamplitude modulated or AM signal according to data to be transmitted.22. The low power receiver according to claim 1, wherein said low powerreceiver is configured to receive three signals with asymmetricalfrequency spacing in order to generate a frequency-shift-keyed or FSKsignal to be transmitted.
 23. The low power receiver according to claim22, further configured to selectively filter a frequency pair F1 and F2,or F2 and F3, and to apply only a desired selected frequency pair to thesecond nonlinear device.
 24. The low power receiver according to claim23, wherein the frequency pair selection is utilizes bandpass filters toselect a desired two signals.
 25. The low power receiver according toclaim 24, wherein the frequency pair selection is utilizes anotch-filter to remove an unwanted signal.
 26. The low power receiveraccording to claim 19 further configured to selectively filter afrequency pair F1 and F2, or F2 and F3, and to apply only a desiredselected frequency pair to the second nonlinear device or whereinbandpass filters are configured to selectively filter and select adesired two signals to apply to the second nonlinear device and whereinthe frequency selection and filtering is included as a part of saidswitch.
 27. The low power receiver according to claim 24 whereinbandpass filters are utilized to selectively filter and select a desiredtwo signals to apply to the nonlinear device, wherein the bandpassfilter or notch filters are utilize high-Q crystal, ceramic, SAW, orMEMS resonators.
 28. The low power receiver according to claim 23,further comprising two different nonlinear devices and configured toselectively switch between said two different nonlinear devicesconnected to said antenna to generate different mixed products thatresult in an FSK-modulated signal to be transmitted.
 29. The low powerreceiver according to claim 1, further comprising an RF amplifierconnected to the second nonlinear device.
 30. The low power receiveraccording to claim 1, further comprising a RF amplifier that isincorporated as part of the second nonlinear device, whereinnonlinearity characteristics of said nonlinear device are utilized togenerate a desired transmit signal.
 31. The low power receiver accordingto claim 1, wherein said nonlinear device and said second nonlineardevice are connected to said antenna via a switch for selectiveconnection of said nonlinear device or said second nonlinear device tosaid antenna.
 32. The low power receiver according to claim 1, whereinsaid receiver is configured to receive at least two pairs of redundanthigh frequency carriers and a first pair of high frequency carriershaving a difference in frequency, which is equal to a difference infrequency of a second pair of high frequency carriers.
 33. The low powerreceiver according to claim 1, wherein said receiver is configured toreceive at least two pairs of high frequency carriers, and a frequencydifference of said at least two pairs of high frequency carriers differfrom each other, and wherein said receiver is further configured togenerate two separate intermediate frequency signals at differentfrequencies from said two pairs of high frequency carriers representingtwo different data signals.